This application claims the benefit of priority from Japanese Patent Application No. 2001-309401, filed on Oct. 5, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an X-ray CT (Computed Tomography) apparatus, particularly to the imaging of a scanogram for positioning the photographing portion or setting the photographic condition, which is conducted preceding the CT image reconstruction process.
2. Description of the Background Art
Generally, a scanogram is an X-ray fluoroscopic image of a portion of a subject in a predetermined range of the subject. When imaging of the subject is conducted by an X-ray CT apparatus, a scanogram is initially obtained and on the basis of this scanogram, the positioning of the slice position or setting of the photographic condition is conducted. At this position, the scanning by the X-ray is then conducted and the CT tomogram is obtained.
For example, as disclosed in Japanese Utility Patent Disclosure (Kokai) 61-82605, the X-ray tube and X-ray detector are not rotated but held stationary. While the top board on which the subject is placed is being moved in the body axial direction of the subject, the X-ray is irradiated. On the basis of the obtained projection data, the scanogram is made.
When the scanning for obtaining the CT tomogram is conducted, the slice position is determined on the basis of the scanogram. Then, after the top board on which the subject is placed is returned to the initial position once, the top board is moved again and the x-ray tube is arranged at the slice position determined to the subject. While the X-ray tube is being rotated around the subject at each position, the X-ray irradiation is conducted. On the basis of the projection data obtained thereby, the tomographic image of the subject at each slice position is obtained.
The above described prior art X-ray CT apparatus is a single-slice CT apparatus. This single-slice CT apparatus has an X-ray tube irradiating a fan-shaped X-ray beam (fan beam), and an X-ray detector in which X-ray detector elements of the M channel (for example, 1000 channel) are arrayed fan-shaped or linearly in one row.
The prior art also includes an X-ray CT apparatus using a helical scan system. While the X-ray tube and the X-ray detector are continuously rotated in the prior art helical scan X-ray CT apparatus, the top board is made to move in the body axial direction (slice thickness direction) of the subject, and the tomographic data of the subject is acquired. Recently, a multi-slice CT apparatus is put to the practical use. The multi-slice CT apparatus has the X-ray tube irradiating the conical X-ray beam (cone beam), and the two dimensional X-ray detector having X-ray detector elements arrayed in slice thickness direction (in a body axial direction) in a plurality of rows, each X-ray detector element row having an array of M-channel elements (M channels times N rows).
In the imaging of the scanogram by the multi-slice CT apparatus, for example, as disclosed in Japanese Patent Disclosure (Kokai) 11-76223, the data output from the X-ray detector element of the X-ray detector is bundled in the row direction, and on the basis of the bundled data, the scanogram for 1 slice thickness (width) of the central position of the X-ray detector is generated.
However, in the imaging of the scanogram by the single slice CT apparatus, only the data for one slice width can be obtained by one imaging. Therefore, it takes a long period of time (for example, about 10 sec) in the imaging of the scanogram whose imaging range is wide. Accordingly, it takes a long period of time when using the scanogram to plan the scanning and, thus, therefore, there is a problem not only to force the burden on the subject (patient), but also to lower the patient throughput. Further, because the imaging is conducted by moving the subject for each slice thickness, an overlap is produced in adjoining slices at every time of the scanning, and there is a possibility to cause the subject to be irradiated with excess X-rays. Further, the time to irradiate the X-ray is prolonged, resulting in the shortening the life of the X-ray tube.
The same problem exists with the multi-slice CT apparatus because only 1 slice width data is obtained by 1 scanning. Thus, it takes a long period of time for the imaging of the scanogram of the necessary range. Therefore, the time to irradiate the X-ray is also undesirably prolonged.
In order to solve such problems, an object of the present invention is to use effectively the x-rays irradiated from the X-ray tube, and to conduct the imaging of a scanogram of wide range in a short time.
According to one aspect of the present invention, there is provided an X-ray CT apparatus including an X-ray tube for generating X-rays; an X-ray detector having a plurality of rows of X-ray detector elements arrayed in a slice thickness direction for detecting X-rays transmitted through a subject; a selector for selecting which of the rows of the X-ray detector elements in the slice thickness direction are necessary for generating a scanogram for a pre-set slice width; and a scanogram processing unit configured to generate the scanogram by using data detected by the row of X-ray detector elements selected by the selector.
According to another aspect of the present invention, there is provided a method of X-ray CT imaging including irradiating X-rays using an X-ray tube; detecting X-rays transmitted through a subject using an X-ray detector having a plurality of rows of X-ray detector elements arrayed in a slice thickness; selecting which of the rows of the X-ray detector elements in the slice thickness direction are necessary for generating a scanogram for a pre-set slice width; and generating the scanogram by using data detected by the selected row of X-ray detector elements.
According to the above-described structure, it is possible that wide range of the scanogram can be obtained in a short time, the burden to the subject is lightened, and the patient throughputs can also be increased.